1. Field of the Invention
This invention relates generally to diode pumped intracavity frequency doubled lasers, and more particularly to continuous wave, diode pumped intracavity frequency doubled lasers that are multiaxial mode lasers which exhibit high amplitude stability.
2. Description of Related Art
Continuous wave ion lasers are relatively reliable sources of continuous wave green laser light with low amplitude noise, and provide output power at the multiple watt level. These devices convert electrical power into optical power with efficiencies of only a small fraction of one percent. There are many applications that would benefit from the development of a highly efficient, low cost, diode-pumped, continuous-wave, solid-state green, blue, red, near infrared, or UV laser source, also at the multiple watt level and with comparable amplitude stability.
Certain fundamental difficulties with intracavity-frequency-doubled solid-state lasers were discovered and numerically modeled in early work by Baer. See for example T. Baer, J. Opt. Soc. Am. B., Vol. 3, No. 9, pp. 1175-1180 (1986), and U.S. Pat. Nos. 4,656,635 and 4,701,929. It was reported and disclosed that large amplitude fluctuations are observed on the green output beam and the intracavity infrared laser beam when a frequency doubling crystal such as KTP is introduced into an otherwise amplitude-stable multiaxial mode diode-pumped Nd:YAG laser. It was also reported that the large amplitude noise on the green output beam disappears when an appropriate etalon is placed in the laser cavity that forces single axial mode oscillation. In the multiaxial mode case, where 2 to 4 modes were oscillating, the green output power was seen to fluctuate with up to 100% modulation depth. Baer's experimental work and theoretical model indicated that the insertion of a frequency doubling crystal in this multiaxial mode laser resulted in nonlinear coupling of the loss of the infrared axial modes via sum frequency generation. A high peak power in one axial mode induced a high nonlinear loss for the other axial modes, and caused an unexpected and undesirable pulsing effect.
As an example of the effect described by Baer, a laser with two infrared axial modes generated three green frequencies; two were doubled modes and the other a sum frequency mode. The sum frequency process couples the two infrared axial modes in a way that can cause them to switch on and off in a sequential fashion. The typical period of this mode coupling was found to be a function of the magnitude of the nonlinear conversion. For weak conversion, the period was short and the modes minimally modulated. For stronger conversion, the mode coupling period lengthened, and the modes switched on and off in pulses of high peak power, completely out of phase with each other in a semi-periodic fashion. The noise spectrum of such a laser typically showed substantial peaks in the 10 to hundreds of kHz range for either the green or infrared, and corresponded to considerable amplitude fluctuations.
A source with this type of amplitude modulation is not as generally useful as one with low amplitude noise, and therefore high amplitude stability. As an example, for applications in ophthalmology, such as retinal photocoagulation, amplitude stability is required on the time scale of the typical exposure durations for accurate control of therapeutic effects. Another example is the use of a green laser as a pump for a second laser, such as a dye or Ti:Al2O3 laser. Deep amplitude modulation at certain frequencies can cause undesirable amplitude modulations on the output of the second laser.
A number of methods for stabilizing the intracavity-frequency-doubled output of a diode-pumped solid-state laser have been described and demonstrated. The most common materials have been Nd:YAG as a laser medium and KTP as a nonlinear, doubling medium. For this reason, the most common type of phase matching is Type II. One technique that has been used in an attempt to stabilize the frequency doubled output from such systems included insertion of intracavity quarter wave plates (see M. Oka, and S. Kubota, Opt. Lett. 13, 805 (1988)). The Oka quarter-wave technique can result in two orthogonally polarized infrared eigenmodes that are not coupled by sum-frequency generation. The Oka configuration was shown to be amplitude-stable under certain conditions. However, for higher output powers this configuration requires the addition of an etalon (M. Oka et al, 1993 Advanced Solid State Laser Conference, paper AMG 1). It was reported that this system could be stable for only a few hours at a time. The temperature control of the KTP is imperative with this technique. Other techniques used for stabilizing the output of intracavity-doubled solid-state lasers include optical cavity temperature control (see U.S. Pat. No. 4,884,277 issued to Anthon, et al. on Nov. 28, 1989) and forcing single frequency operation (see U.S. Pat. No. 5,164,947 issued to G. J. Lukas, et al. on Nov. 17, 1992, and W. Weichmann et al., 1995 Advanced Solid State Laser Conference, papers TuD4 and WD4).
Another method of achieving low noise operation is also based upon single frequency operation: J. Nightingale et al have developed an intracavity-doubled unidirectional ring laser with diode-pumped Nd:YAG and KTP (U.S. Pat. Nos. 5,052,815, 5,136,597, and 5,170,409, and 1994 Compact Blue Green Laser Conference, Post-deadline paper PD6).
While all of these techniques have demonstrated regimes of operation where the frequency doubled output is measured to have low amplitude noise, in all cases the techniques are difficult to implement in a reliable, low cost fashion that is resilient to changes in environmental conditions, such as temperature. The techniques employed typically must maintain an inherently amplitude-unstable system within the narrow range of parameter space for which the system is stable. The single frequency intracavity-doubled systems can suffer mode-hops that result in undesirable discontinuities in output power. To avoid this, single frequency systems must be designed to be resistant to such mode hops. Additionally, the potential for scaling currently available systems to higher power may be limited.
It would be highly desirable to provide an inherently amplitude-stable, intracavity-frequency-doubled, solid-state laser that does not require active stabilization or single axial mode operation. Additionally, there is a need for a laser of this type that remains stable over a range of environmental conditions, such as changes in ambient temperature. There is a need for an intracavity-frequency-doubled laser that does not exhibit discontinuities in output power, as with single frequency systems. There is also a need for an amplitude stable, intracavity frequency doubled laser that can be scaled to higher powers. A similar frequency-tripled system would also be useful.